A process for preparing aliphatic bromides in which the bromine atom is attached to a terminal carbon atom is described in Brit. 841,745, published Jul. 20, 1960, all disclosure of which is incorporated herein by reference. The process comprises passing hydrogen bromide and molecular oxygen into a solution of a terminal olefin (CH2═CR1R2, where R1 is hydrogen, alkyl, or haloalkyl, and R2 is alkyl, or haloalkyl) in the aliphatic bromide product. The molar proportion of hydrogen bromide in solution in not substantially in excess of that of the olefin used, and preferably is a molar proportion of olefin to hydrogen bromide of at least 1:1. The volume of molecular oxygen passed through the reaction mixture identified is between 0.01-1 mole of oxygen per mole of hydrogen bromide fed. Air is identified as a suitable source of oxygen.
The production of n-propyl bromide by free-radical (anti-Markovnikov) addition of HBr to propylene using air typically results in a selectivity of about 97% for n-propyl bromide (NPB), with >99.5% reaction completion. Typically some isopropyl bromide (IPB) is co-produced and this is of use as an article of commerce. However, in such processing, trace levels of elemental bromine are formed the reaction. Studies in our laboratories have shown that formation of trace amounts of bromine and peroxy radicals leads to the formation of small amounts of various other by-products such as 1,2-dibromopropane, acetone, bromoacetone, propionaldehyde, 1,3-dibromoacetone, 1-propanol, and 2-propanol. The formation of acetone and bromoacetone, typically at levels of about 200 ppm, is especially undesirable as bromoacetone even at such low concentrations is a powerful lachrymator. Also when it is desired to recover isopropyl bromide as a co-product, acetone is troublesome as it readily dissolves in isopropyl bromide thus necessitating another separation step.
In the case of C4 and higher aliphatic bromides in which the bromine atom is attached to a terminal carbon atom, formation of analogous undesirable impurities during their production by hydrobromination is also deemed disadvantageous.
It would be of considerable advantage if a way could be found of minimizing or suppressing the formation of such undesirable by-products, thereby enabling recovery of high quality product such as NPB, even if by-product HBr feedstock containing some HCl is used in the process. It would also be advantageous if the way of accomplishing these goals would likely be applicable to use of the same technology in the production of other aliphatic bromides. This invention is believed to provide an efficient and effective way of achieving these goals.